We are engaged in the study of hematopoietic stem cell differentiation. Specifically, we are investigating the mechanisms by which the stroma of hematopoietic organs induces pluripotential stem cells to differentiate into committed erythroid, granulocytic and megakaryocytic precursors. Using an in vivo experimental system of ectopic implantation of hemopoietic stem cells, we are attempting to isolate stromal components which are then introduced into the system and assayed for their erythroid inductive capacity. Preliminary histological evidence suggests that stromal cells secrete short range diffusible factors which induce pluripotent stem cells to differentiate into committed stem cell lines. We will attempt to isolate such factors from the active stromal components. Concurrently, we are studying the mechanisms of erythroblast maturation in normal and "stress" situations and , in particular, the origin of stress macrocytes. Fractionation and isolation of normoblasts and reticulocytes at various stages of maturation will be followed by volume, size and hemoglobin determinations on cell fractions as well as on individual cells.